Project Description The overall objective of this research proposal is to explore and characterize partner and relationship characteristics that place young men who have sex with men (YMSM) at risk for HIV infection. In 2008, YMSM represented 68% of infections among all adolescents and young adults and were one of the only HIV risk groups showing an increasing rate of infections. Among YMSM, nearly 80% of new infections occurred with main partners. While sexual HIV transmission most often occurs in dyads, little is known about the romantic and sexual relationships of YMSM. No evidence-based, couples HIV prevention interventions exist for MSM and there are limited effective behavioral interventions for YMSM. Key basic research is needed before intervention development can begin. There are four specific aims in this proposal: Aim 1 is to assess relationship and partner characteristics across development and their link to HIV risk for YMSM. To do this we will extend a unique, longitudinal study (Project Q2) that includes an ethnically diverse sample of 122 YMSM previously followed for 2.5 years with excellent retention. Two additional waves of data collection will assess relationship processes (e.g., formation, duration, quality, power dynamics), partner characteristics (e.g., age, HIV status), and risk behaviors (e.g., unprotected sex) on a partner-by-partner basis. We will use this longitudinal partnership-level data to conduct innovative modeling of relationship and partner characteristics related to HIV risk in the same individual across multiple relationships. Aim 2 is to explore a theoretical model (i.e., IMB) adapted for understanding attitudes and cognitions that contribute to HIV risk behaviors among YMSM in relationships. Aim 3 is to address logistical and ethical issues associated with conducting couples- based interventions from the perspective of YMSM. To achieve this, we will conduct qualitative interviews with male couples (n = 20) recruited from Project Q2 participants who report having a main partner. Thematic analysis of the data will yield insight into the development and implementation of a dyadic HIV prevention intervention for YMSM. Aim 4 is to analyze study findings with our team of expert consultants to inform a grant application to develop and test an HIV prevention program for young male couples. The long-term goal of this proposed research is the development of an innovative couples HIV prevention intervention for YMSM that may have a high impact in reducing the number of new transmissions among this vulnerable population.